The electromagnetic spectrum is used by a variety of different wireless communication systems, such as terrestrial wireless communication systems, satellite communication systems, broadcasting services and radio navigational systems. To prevent signal interference between different transmissions, access to specific portions of the wireless spectrum is regulated by national and international policies. The most common form of spectrum regulation is that different parts of the radio spectrum are allocated to different types of services, with this allocation typically lasting for several decades.
Over the last twenty years there has been a tremendous proliferation of certain wireless communication systems such as cellular telephone systems and wireless data networks. This growth has led to a large demand for use of the limited spectrum resources that have been allocated to these systems. Moreover, the predicted spectrum capacity demand for the coming decades by far exceeds what can be provided by the limited spectrum bands allocated to current cellular and wireless data systems. At the same time, spectrum resources assigned to certain other systems are only sparsely used. Consequently, new paradigms of dynamic spectrum access have been investigated in recent years by regulatory and standardization bodies. For example, the US Federal Communications Commission ruled in 2008 to allow secondary communication systems to operate within the television (TV) broadcast spectrum. In this approach to dynamic spectrum access, secondary systems are permitted to access spectrum resources that have been allocated to a primary system, provided that the secondary system does not harmfully interfere with the primary service. However, the condition that the secondary system cannot cause noticeable degradation of the primary service creates various technical challenges, and various approaches have been developed to address these challenges.
Primary-secondary spectrum sharing can be implemented in three different ways: underlay, interweave and overlay operation. In underlay operation, the secondary system transmits at a signal level that is below an interference level that is close to the noise level, thereby ensuring that the secondary transmission remains mostly unnoticed by the primary system. This approach has little benefit for most systems such as cellular communication systems and is typically used in local short-range ultra-wideband systems. In interweave operation, the secondary system intelligently determines spectrum holes or spectrum white spaces that remain unused by the primary system in time, frequency and/or geographic location. This strategy is based on cognitive radios that can detect the usage of spectrum resources in order to discover spectrum usage opportunities. In the interweave approach, cognitive radios use their capabilities solely for dynamic access to unused frequency bands. To operate a network close to its capacity limits, this approach is too restrictive.
The overlay approach relaxes the assumption of orthogonal transmissions, and attempts to exploit cognition more generally for cooperation, precoding against interference and interference cancellation. In overlay operation, the secondary system is assumed to know, in the most ideal case, in advance the message that is transmitted by the primary transmitter, as well as the codebook of the primary system. This allows the secondary system to design its own transmitted signal such that interference from the primary system to the secondary receiver can be mitigated. At the same time, the secondary system can cooperate with the primary system by relaying the primary signal; this enables the secondary system to compensate for interference that it causes to the primary receivers.
Theoretical results indicate that the cognitive radio techniques used in the overlay approach should provide a valuable capacity extension for a secondary system. In practice, the achievable benefits strongly depend on the channel characteristics. For example, in certain practical scenarios the benefits of implementing cognitive transmission for overlay operation may be negligible, whereas in other scenarios a large gain may be achieved. Blindly applying cognitive transmission for a secondary system is not necessarily beneficial. Currently, knowledge is lacking on when cognitive transmission may be useful. Moreover, for cognitive transmission it has been typically assumed in prior work that perfect channel information of all the channel gains is available at the transmitter. In systems where such channel information is not available, prior techniques for cognitive transmission are not applicable, or provide significantly reduced performance. The use of cognitive transmission is further complicated in the case of a primary broadcast system since there are multiple receivers of the primary broadcast signal, and in this scenario it is not possible to apply the cognitive transmission methods proposed in prior work.
In view of the above, there is clearly a need for solutions to the outstanding challenges in realizing cognitive transmission overlay operation, especially in the case where the primary communication system is a broadcast system.